In the recovery of oil from oil-bearing reservoirs, it is typically possible to recover only minor portions of the original oil in place by primary recovery methods which utilize only the natural forces present in the reservoir. Thus a variety of supplemental techniques have been developed and used to increase oil recovery. A commonly used secondary technique is waterflooding which involves injection of water into the oil reservoir. As the water moves through the reservoir, it displaces oil therein to one or more production wells through which the oil is recovered.
One problem encountered with waterflooding operations is the relatively poor sweep efficiency of the water, i.e., the water can channel through certain portions of the reservoir as it travels from the injection well(s) to the production well(s), thereby bypassing other portions of the reservoir. Poor sweep efficiency may be due, for example, to differences in the mobility of the water versus that of the oil, and permeability variations within the reservoir which encourage flow through some portions of the reservoir and not others.
Various enhanced oil recovery techniques have been used to improve sweep efficiency. One such technique involves increasing the viscosity of the water using non-biodegradable thickening agents such as polyvinyl aromatic sulfonates as described in U.S. Pat. No. 3,085,063. U.S. Pat. No. 4,678,032 describes a method for treating a subterranean formation by injecting a polymer solution which includes an essentially non-ionic polymer selected from the group consisting of poly(vinylalcohol-co-vinylcarboxyl) and poly(vinylalcohol-co-vinylether), which is crosslinked with a transition metal selected from Groups IIIa-VIa, VII, and Ib-Vb of the Periodic Table to form a gel. Such cross linked polymer systems require a crosslinker and monomer/polymer in at the current concentration, at the same location and at a temperature appropriate for crosslinking to occur deep in the reservoir. Such conditions are difficult to achieve in reality. Furthermore, the crosslinkers are often expensive for the amounts required and the polymer system are not readily dissolvable in cold ocean water where it may be required, for example, on a oil production platform at sea.
There is therefore a need for a method to improve sweep efficiency using cost-effective, biodegradable materials that exhibit shear-thinning properties and thus exhibit lower viscosity during injection and increased viscosity in the oil reservoir. The present invention provides such a method. Another aspect of this invention is to provide a polymer for this application that is readily soluble in cold sea water. Application of such a polymer would be an advantage for operations on an oil platform at sea (for example, the North Sea), since the water used for flooding the oil reservoir is cold sea water and there are no or limited resources on typical platforms to heat up sea water to help dissolve the polymer.